We plan to study regulatory events in early hematopoiesis in humans. In order to do this we will establish assays in culture for human pluripotent stem cells as well as develop techniques which permit the long-term maintenance and renewal of such stem cells in culture. Growth from normal human marrow cells will be compared with findings in cultures of cells from patients with various myeloproliferative syndromes and others with marrow failure such as aplastic anemia. The cellular interactions which influence maintenance of long-term cultures will be assessed by monoclonal antibodies directed to lymphocyte subpopulations. In other studies, we plan to characterize those factors which promote the growth of early hematopoietic cells. A variety of antibodies to these growth factors will be developed by using the hybridoma technique. The overall goal of this portion of the work is to distinguish between pathway-specific factors for granulopoiesis and erythropoiesis and, if successful, to assess the physiological relevance of such factors in vivo. In studies in humans, we will compare and quantitate mechanisms of hematopoietic regulation in myeloproliferative syndromes. These will include the response in vivo and in vitro of erythroid colony-forming cells to changing levels of erythropoietin and the application of specific antibodies, able to distinguish between the major isoenzymes of glucose 6-phosphate dehydrogenase, to the study of normal and neoplastic cell proliferation in response to stimulation. Through these studies, we hope to learn more about the regulatory interactions which influence the phenotypes of the myeloproliferative syndromes in humans.